docs: def render_field_error(self, obj_id, obj, exception, request):
          """
          Default rendering for items in field where the the usual rendering
          method raised an exception.
          """
          if obj is None:
              msg = 'No match for ID={0}'.format(obj_id)
          else:
              msg = unicode(exception)
          return u'<p class="error">{0}</p>'.format(msg)